Current climate change is a major threat to marine biodiversity with severe effects on marine ecosystem function and stability. Phytoplankton, at the base of marine food webs, already start showing shifts in species composition and abundance as a consequence. Yet it is unclear how such changes occurred in the past and likely continue in the future. PHYTOARK looks into the Holocene history of the Baltic Sea to estimate patterns of phytoplankton biodiversity in the past and to assess its ecosystem function under global change. The project will address the questions: 1. How did primary producer communities react to rising temperatures and changing salinities during Holocene warm periods? 2. Did populations of key species change over time? 3. Does climate change and human impact accelerate these changes? 4. How will phytoplankton of the future function? We answer these questions through an integrative approach that combines the newest advances in environmental DNA time series, paleo-genomics, molecular organic proxies, resurrection ecology and ecological modelling. We will analyse dated sediment cores capturing ~8000 years of Baltic history to reconstruct phytoplankton composition and function based on eDNA and trait information, document evolutionary adaptation and trait changes of populations on resurrected individuals and eDNA signatures, project future phytoplankton biodiversity and function in models and define reference status variability to aid environmental assessment efforts. PHYTOARK places eco-evolutionary changes of phytoplankton into a multi-millennial context to understand, predict and monitor the consequences of global change in marine ecosystems. Connecting excellence in biodiversity, climate change, paleo-ecology and marine system science, the network will generate a new association of Leibniz Institutions and Universities that can significantly advance an integrative understanding of human impact on marine biodiversity and ecosystem function.